This invention relates generally to speed gear reducer drive assemblies and, more particularly, to a novel gear reducer assembly for use on the boom of aerial lift trucks. The novel assembly includes a primary worm gear set and a secondary spur gear set mounted within a common one-piece housing.
In aerial lift trucks, a top boom plate is rotatably mounted with respect to a lower truck plate by way of an intermediate turntable bearing. The top boom plate is connected to the inner race of the turntable bearing and the outer race is fixed to the truck plate. Currently, there are two drive configurations used in the market place for rotating the boom plate on aerial lift trucks. Each of those has distinct drawbacks. In one drive configuration, a gear reducer is externally mounted on the top boom plate and includes a drive spur pinion which extends downwardly through the boom plate and drivingly engages spur gear teeth formed on the external face of the large diameter outer race of the turntable bearing. This type of drive assembly encounters a number of problems. For example, the top surface of the boom plate must be machined to accommodate a mounting plate for the reducer. The overhung drive pinion often causes misalignment and uneven wear on both the pinion and the turntable bearing, and holding the reducer down to resist rotation requires the use of jack bolts.
Another drive configuration, known as a swing drive, includes a worm gear reducer mounted directly between the boom plate and the truck plate, the
Another drive configuration, known as a swing drive, includes a worm gear reducer mounted directly between the boom plate and the truck plate, the worm engaging against a helical gear formation on the external surface of the outer race of the turntable bearing. This configuration is substantially less expensive than the externally mounted reducer, but suffers from low torque rating, poor life due to point contact between the worm and turntable bearing, causing high wear on the turntable bearing due to high contact stresses. In addition, performance problems such as backlash and "slip sticking" occur when turning down a slope due to high friction and loads bearing on the conventional steel worm and steel helical gear.
The combined spur/worm swing drive assembly of the invention was developed to overcome those problems.